An integrated circuit is a complete electronic circuit, containing transistors, diodes, resistors, and capacitors, along with their interconnecting electrical conductors, contained entirely within a single chip of silicon. Integrated circuits continue to decrease in size, and the circuits they contain continue to increase in complexity. This increases the opportunity for defective chips resulting from a failed element or a defective conductor. The complexity of these devices and the need to interconnect the circuits create very narrow performance tolerances. One way these needs have been met is to manufacture fuses into the device. Fuses can be opened to isolate defective areas and allow the rest of the circuit to be used. Fuses can also be used to trim a circuit, enable a particular mode, or enable or disable different segments of the circuit. By using fuses integrated circuit manufacturers are able to reduce the amount of semiconductor scrap. The continuous drive to reduce the overall size of integrated circuits creates a need to design fuses and other elements of integrated circuits in such a way as to minimize the space they require.
Another way to reduce semiconductor scrap is to provide redundant elements on integrated circuits. If a primary element is defective a redundant element can be substituted for that defective element. One example of an integrated circuit device which uses redundant elements is electronic memory. Typical memory circuits comprise millions of equivalent memory cells arranged in addressable rows and columns. By providing redundant elements, defective memory cells can be replaced. Because the individual primary memory cells of a memory are separately addressable, replacing a defective cell typically comprises opening fuse-type circuits to `program` a redundant cell to respond to the address of the defective primary cell. This process is very effective for permanently replacing defective primary memory cells.
Circuit designers continuously strive to achieve higher population capacities without a corresponding increase in physical size. Reducing the size of individual elements in integrated circuits is one way in which available die real estate is maximized. For example, as memory density increases the number of fuses needed for redundancy in a given memory device also increases. A 256M DRAM is expected to have more than 10,000 laser fuses. Most components of the memory devices can be scaled to meet the space restrictions resulting from the higher densities. However, laser fuses used to implement redundancy can not be scaled due to mechanical restrictions related to current laser technology. Fuse width must be kept large enough to cover the laser spot so that the fuse can absorb a large quantity of heat. In addition, the fuse-to-fuse space must be kept large enough to allow for mechanical laser alignment tolerances and to prevent unintentional programming of a fuse adjacent to an exploding fuse. These laser alignment tolerances, as well as the requirements for a large passivation opening, limit the length of the fuse. Currently the constraints dictated by the laser repair requirements limit the fuse pitch to about 3 microns. The demand for increasing numbers of fuses combined with the fixed pitch limitation create a need for improvements in the laser fuses.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a design which provides an increased density of laser fuses.